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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
/* A type-erased callable wrapper. */
#ifndef mozilla_Function_h
#define mozilla_Function_h
#include "mozilla/Attributes.h" // for MOZ_IMPLICIT
#include "mozilla/Move.h"
#include "mozilla/RefCounted.h"
#include "mozilla/RefPtr.h"
// |function<Signature>| is a wrapper that can hold any type of callable
// object that can be invoked in a way that's compatible with |Signature|.
// The standard "type erasure" technique is used to avoid the type of the
// wrapper depending on the concrete type of the wrapped callable.
//
// Supported callable types include non-member functions, static member
// functions, and function objects (that is to say, objects with an overloaded
// call operator; this includes C++11 lambdas). Member functions aren't
// directly supported; they first need to be wrapped into a function object
// using |std::mem_fn()| or an equivalent.
//
// |Signature| is a type of the form |ReturnType(Arguments...)|. Syntactically,
// this is a function type; it's not used in any way other than serving as a
// vehicle to encode the return and argument types into a single type.
//
// |function| is default-constructible. A default-constructed instance is
// considered "empty". Invoking an empty instance is undefined behaviour.
// An empty instance can be populated with a callable by assigning to it.
//
// This class is intended to provide functionality similar to the C++11
// standard library class |std::function|.
namespace mozilla {
namespace detail {
template<typename ReturnType, typename... Arguments>
class FunctionImplBase : public mozilla::RefCounted<FunctionImplBase<ReturnType, Arguments...>>
{
public:
MOZ_DECLARE_REFCOUNTED_TYPENAME(FunctionImplBase)
virtual ~FunctionImplBase() {}
virtual ReturnType call(Arguments... aArguments) = 0;
};
// Normal Callable Object.
template <typename Callable, typename ReturnType, typename... Arguments>
class FunctionImpl : public FunctionImplBase<ReturnType, Arguments...>
{
public:
explicit FunctionImpl(const Callable& aCallable)
: mCallable(aCallable) {}
ReturnType call(Arguments... aArguments) override
{
return mCallable(Forward<Arguments>(aArguments)...);
}
private:
Callable mCallable;
};
// Base class for passing pointer to member function.
template <typename Callable, typename ReturnType, typename... Arguments>
class MemberFunctionImplBase : public FunctionImplBase<ReturnType, Arguments...>
{
public:
explicit MemberFunctionImplBase(const Callable& aCallable)
: mCallable(aCallable) {}
ReturnType call(Arguments... aArguments) override
{
return callInternal(Forward<Arguments>(aArguments)...);
}
private:
template<typename ThisType, typename... Args>
ReturnType callInternal(ThisType* aThis, Args&&... aArguments)
{
return (aThis->*mCallable)(Forward<Args>(aArguments)...);
}
template<typename ThisType, typename... Args>
ReturnType callInternal(ThisType&& aThis, Args&&... aArguments)
{
return (aThis.*mCallable)(Forward<Args>(aArguments)...);
}
Callable mCallable;
};
// For non-const member function specialization of FunctionImpl.
template <typename ThisType, typename... Args, typename ReturnType, typename... Arguments>
class FunctionImpl<ReturnType(ThisType::*)(Args...),
ReturnType, Arguments...>
: public MemberFunctionImplBase<ReturnType(ThisType::*)(Args...),
ReturnType, Arguments...>
{
public:
explicit FunctionImpl(ReturnType(ThisType::*aMemberFunc)(Args...))
: MemberFunctionImplBase<ReturnType(ThisType::*)(Args...),
ReturnType, Arguments...>(aMemberFunc)
{}
};
// For const member function specialization of FunctionImpl.
template <typename ThisType, typename... Args, typename ReturnType, typename... Arguments>
class FunctionImpl<ReturnType(ThisType::*)(Args...) const,
ReturnType, Arguments...>
: public MemberFunctionImplBase<ReturnType(ThisType::*)(Args...) const,
ReturnType, Arguments...>
{
public:
explicit FunctionImpl(ReturnType(ThisType::*aConstMemberFunc)(Args...) const)
: MemberFunctionImplBase<ReturnType(ThisType::*)(Args...) const,
ReturnType, Arguments...>(aConstMemberFunc)
{}
};
} // namespace detail
// The primary template is never defined. As |Signature| is required to be
// of the form |ReturnType(Arguments...)|, we only define a partial
// specialization that matches this form. This allows us to use |ReturnType|
// and |Arguments| in the definition of the specialization without having to
// introspect |Signature|.
template<typename Signature>
class function;
template<typename ReturnType, typename... Arguments>
class function<ReturnType(Arguments...)>
{
public:
function() {}
// This constructor is implicit to match the interface of |std::function|.
template <typename Callable>
MOZ_IMPLICIT function(const Callable& aCallable)
: mImpl(new detail::FunctionImpl<Callable, ReturnType, Arguments...>(aCallable))
{}
MOZ_IMPLICIT function(const function& aFunction)
: mImpl(aFunction.mImpl)
{}
MOZ_IMPLICIT function(decltype(nullptr))
{}
// Move constructor and move assingment operator.
// These should be generated automatically, but MSVC doesn't do that yet.
function(function&& aOther) : mImpl(Move(aOther.mImpl)) {}
function& operator=(function&& aOther) {
mImpl = Move(aOther.mImpl);
return *this;
}
template <typename Callable>
function& operator=(const Callable& aCallable)
{
mImpl = new detail::FunctionImpl<Callable, ReturnType, Arguments...>(aCallable);
return *this;
}
function& operator=(const function& aFunction)
{
mImpl = aFunction.mImpl;
return *this;
}
function& operator=(decltype(nullptr))
{
mImpl = nullptr;
return *this;
}
template<typename... Args>
ReturnType operator()(Args&&... aArguments) const
{
MOZ_ASSERT(mImpl);
return mImpl->call(Forward<Args>(aArguments)...);
}
explicit operator bool() const
{
return bool(mImpl);
}
private:
// TODO: Consider implementing a small object optimization.
RefPtr<detail::FunctionImplBase<ReturnType, Arguments...>> mImpl;
};
template<typename Signature>
bool
operator==(const function<Signature>& aX, decltype(nullptr))
{
return !aX;
}
template<typename Signature>
bool
operator==(decltype(nullptr), const function<Signature>& aX)
{
return !aX;
}
template<typename Signature>
bool
operator!=(const function<Signature>& aX, decltype(nullptr))
{
return bool(aX);
}
template<typename Signature>
bool
operator!=(decltype(nullptr), const function<Signature>& aX)
{
return bool(aX);
}
} // namespace mozilla
#endif /* mozilla_Function_h */
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